Electron multiplier



Dec. 12, 1939. w, c s c, ET AL 2,183,309

ELECTRON MULTIPLIER Filed March 17, 1938 Patented Dec." l"2, 1939- I j 1, ,-.[um1 -1-:o STATES nwc'mon MULTIPLIER Wei-1m- Fresnel. Berlin-Oharlottenburg, and Rudolf Belme, Berlin-Lichterfelde, Germany,

usignorstotheflrmof Fernseh Aktien-G'esell schaft. Zehlendorl, near Berlin, Germany Application March 17, 1938, Serial No. 196,496

In Germany March '19, 1931- 7 10 Claims. (01. 250-150) This invention relates .to electron. multipliers and particularly to multipliers in which the number of primary electrons is varied in accordance with a signal. f, Tubes have been constructed hitherto in which a beam of primary electrons of defined. cross section is deflected across an aperture of aidiaphragm or across a boundary 'of a-diaphragm,

in accordance with a signal, whereby the'number of primary electrons allowed to pass :the diaphragm is varied correspondingly. .Such tubes, however, were extremely susceptible to'interference due to stray extraneous fields. In the construction of these tubes little consideration was given to the necessity for obtaining a clearly defined cross section of the primary electron beam in the plane of the diaphragm across which it was deflected. Therefore, the results obtained. from these tubes were. quite unsatis- D factory.

It is the object of the present invention to provide an improved electron multiplier, which will overcome the disadvantages of arrangements of the prior art.

According to the present invention, an electron image of a clearly-defined cross section of an electron beam is produced by means of electrostatic lenses and this-electron image is deflected by means of a deflecting system, over a l boundary located in the plane of the electron image. The clearly-defined cross section of the electron beam can be gained by means of a suitable aperture in a diaphragmagainst which the' initial stream of electrons isdirected. Another i possibility is that the smallest cross section of the electron beam in front of. the cathode, i. e., the first cross-over, is used for electron-optical reproduction. It suflices even that the reproduced cross section is clearly defined andpossesses sharp outlines on one side only. Gener- I ally a uniform intensity of the portion of the reproduced cross section used-for intensity controlis desirable. 4 e

In the following, an embodiment'of the :invention shall be described with which good experimental results'have been obtained. Anelectron image of a cross sectionof an electron beam of substantiallyuniform electron density is produced by means of cylindrical electrodes forming an electron optical lens system. The undeflected position of this electron image lies in close proximity to an aperture in a diaphragm .disposedin the plane of the electron image. In operation, the electron image is deflected across the aperture, thereby varying the amount of elec- -in the trons passingthrough the aperture into an electron multiplier. The amplitudes of the deflecting voltage, which is simultaneouslythecontrol .voltage, are chosen to be so'small that only a smalljportion of the electronimage is deflected 5 over-the aperture? The amplitude of deflection plane of the'electron image may be, for instance, only 1/100 mm,

The use of an electrostatic lens for the develop ment of the electron image is preferable because of the lack of current flow therein which increases the simplicity in filtering means over magnetic lenses.- In order to obtain high sen sitivity, a shield against extraneous magnetic 'fields must be provided. For this purpose, it is preferred to use an iron shield, which may in part consist of windings of iron wire and which is subjected to an annealing process. In view of the fact that 'an iron screen or shield is necessary, it is preferred to use an electrostatic lens 2 system, since the placing of amagnetic electron optical system inside the iron shield would have considerabledisadvantages. The combination of a screen against extraneous magnetic fields and an electrostatic lens system highest sensitivity obtainable.

A preferred embodiment of the invention is disclosed in the drawing, wherein Fig. 1 is a viewin section longitudinally of the tube.

Fig. 2 is a view in section along the linear-2 of Fig. 1.

Fig. 3 is a view of Fig.- 1. I I

In the drawing, Fig. 1 shows a vacuum recepin section along the line 3-3 tacle I housing a cathode 2, which is preferably indirectly heated and held at a comparatively low temperature. In the present case this cathode is an elongated oxide cathode with its axis normal to the axis of the tube. Two plates, 3 and l, are disposed on the sides of the cathode, which plates replace the conventional concentrating cylinder. In this arrangement a separate-lead from eachof the plates 3 and l is brought out so that different direct current 120- tentials may be applied to them. In this manner, it is possible to focus theemission from the cathode upon an aperture in a diaphragm 5. This arrangement corresponds inprinciple to that of a cylindrical'lens. The aperture in the diaphragm Spossesses the shape of an elongated rectangle, and is so small that the electron den-- sity of the electron beam passing through it is substantially uniform. Between the diaphragm 5 and the plates land 4 a screeningelectrode 6 is disposed, possessing a larger aperture, which .alone renders the 25- is held at the same potential as the diaphragm 5 and a cylindrical electrode I. This prevents the field between the diaphragm 5 and the oathode 2 from reaching through the aperture of 5 the diaphragm 5 and, thereby, distorting the electron image.

The electron-optical system consists of a series of three cylinders, I, 8 and 9, which are provided with annular discs at their ends. These 10 cylinders are aligned along the tube axis and are so dimensioned that the magnification factor with which the electron image is created in the plane I0 is approximately unity, or smaller.

. The space between the electrodes 9 and I0 is taken up by deflection fields generated by potentials applied to the deflecting plates II and i2.

The electrons travel through this space with constant velocity, because electrodes 9 and III are energized at the same potential and the plates II and I2 possess a potential only slightly different.

.flecting plates H and I2 decreases in the direction of the electron path. In this manner, great- :5 er sensitivity is obtained because the shape of the deflecting plates is exactly matched to the boundary surfaces of the electron beam. The

potential applied to electrodes 9 and I0 may be approximately 1,000 volts positive with respect to the cathode. Because of the very weak defleeting voltages (smaller than one millivolt), it

is naturally preferable to provide the interior of the vacuum receptacle with an electrostatic screen for prevention of wall charges. This screen can be omitted in the adjacent multiplier structure. The size of the aperture in electrode I0 is accurately matched to the electron image of the aperture in the diaphragm 5, which is produced in the plane of electrode [0. It is, how- ,ever, preferable to make this aperture as narrow as possible because it is then possible to prevent interfering electrons from entering the multiplying chamber. The long side of the aperture in electrode I0 can be made smaller than the long side of the aperture in the diaphragm 5. in

order to cut off distortions in the electron image which may appear at the corners of the rectangular image at high densities of primary current.

It may appear preferable to dispose one or several electrodes of similar shape in back of the electrode III, to which a lower potential is applied. The drawing shows two such electrodes.

[3 and I4. The electrode l3 nearer the cathode is held at apotential of 980 volts, whereas the electrode ll is held at a potential of only 50 volts. The electrode l3 prevents secondary electrons which are liberated from the electrode l0 by unutilized portions of the electron beam from passing through the aperture in the electrode I0.

These secondary electrons cannot travel against a. potential lower by 20 volts because of their low initial velocity. Furthermore, this electrode prevents the field created by electrode ll from reaching the vicinity of the aperture in electrode. I0

where this could lead to distortions in the elec tron image. Electrode l4 prevents the passage of secondary electrons into the multiplying chamber which are liberated from the electrode 5 and pass through its aperture into the deflecting space. Ifthe electrode 5 is held at a potential of '70 volts, secondary electrons liberated therefrom are so highly decelerated in the plane of the disc I4 that their passage into the multiplying chamber is prevented.

The electrode M can simultaneously be used ing it with high frequency currents. Contrary to conventional practice in cathode ray tubes, the distance between the deas the first secondary emitting electrode of the multiplier system by covering its opening with a wire meshwork capable of secondary emission. The impacting velocity of the electrons emitted by .the cathode and passing through the aperture in the electrode I0 is approximately 50 volts and is sufficient for secondary emission at a ratio greater than unity. Any known type of electron multiplier can be located behind electrode M. In the present case, a number of secondarily emissive grids 18 are shown in back of which a collector plate I! is disposed.

The diaphagm I0 is preferably so mounted that it can be moved or rotated in order to align it to the exact position of the electron image. For this purpose, it may be held by a drop of metal of low melting point (for instance, lead or tin) which is held in a smaller receptacle l5. If necessary, this metal drop can be melted by heat- The position of the diaphragm can then be adjusted in vacuo by means of the force of gravity. It is then held in the new position by the congealed metal. The remaining electrodes can be fixed upon glass rods 16 and I1. It is necessary that all electrodes consist of completely non-magnetic material.

Experiments have shown that a characteristic of good linearity can be obtained with the described arrangement and that the obtainable mutual conductance is greater than in any other known arrangement.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. Electron multiplier, comprising means for generating a primary electron stream, means for producing an electron image of a defined cross section of said electron stream in a plane, 9. diaphragm in said plane possessing an aperture having dimensions substantially proportional to the cross-sectional dimensions of said beam image and an area approximately equal to said beam image, means for deflecting said electron image across said aperture, means for amplifying by electron multiplication that portion of said electron stream passing through said aper ture, and means for collecting the amplified porand means for collecting the amplified portion of 1 said electron stream.

3. Electron multiplier, comprising an elongated oxide cathode for generating a primary electron stream, a diaphragm in front of said cathode possessing a rectangular aperture, the axis of said cathode being parallel to the longitudinal axis of said rectangular aperture, means for producing an electron image of the cross section of said electron stream passing through said rectangular aperture, in a plane, a second diaphragm in said plane possessing an aperture, means for deflecting said electron image across said last-named aperture, means for amplifying by electron mul-- I tiplication that portion of said electron stream' passing through said last-named aperture, and

means for collecting the amplified portion of said electron stream.

4. Electron multiplier, comprising means for generating a primary electron stream, means for producing an electron image of a defined cross section of said electron stream in a plane, a dia-.'

5. Electron multiplier, comprising means forgenerating a primary stream of electrons, means for selecting a cross section of said electron stream, said means comprising a diaphragm possessing an aperture and means for producing a substantially equipotential space about said aperture, means for producing an electron image of said cross section in a plane, a second diaphragm in said plane possessing an aperture, means for deflecting said electron image across said lastnamed aperture, means for amplifying by electron multiplication that portion of the electron stream passing through said last-named aperture, and means for collecting the amplified portion of said electron stream.

6. Electron multiplier, comprising means for generating a primary stream of electrons, concentrating means comprising a plurality of electrodes positioned about said electron generating means, means for selecting a cross section of said electron stream, means for producing an electron image of a defined cross section 01' said electron stream in a plane, a diaphragm in said plane possessing an aperture, means for deflecting said electron image across said laperture, means for amplifying by electron multiplication that portion of said electron stream passing through said aperture, and means for collectingthe amplified portion of said electron stream.

'7. Electron multiplier, comprising means for generating a primary electron stream, means for selecting a defined cross-sectional portion of said electron stream and producing an electron image of said defined cross sectional portion in a plane,

a diaphragm in said plane possessing an aperture having an area of the order of said defined cross- ,sectional portion, means for deflecting said electron image across said aperture, means for aniplifying by electron multiplication that portion of said electron streampassing through said aperture, and means for collecting the amplifiedportion of said electron stream.

8. Electron multiplier, comprising means for generating a primary electron stream, means for producing an electron image of a defined cross section of said electron stream in a plane, a diaphragm in said plane possessing an aperture, means for deflecting said electron image across said'aperture, means for amplifying by electron multiplication that portion of said electron stream passing through said aperture, means-located between said apertured diaphragm and said electron multiplying means for preventing undesirable secondary electrons from entering the multiplication chamber, and means for collecting the amplified portion of said electron stream.

9. Electron multiplier as set forth in claim 1, possessing an electrostatic shield for prevention of wall charge.

10. The method of electron multiplication, comprising the steps of generating a primary stream of electrons, selecting-a cross section of said stream of uniform electron density, pro- .ducing an electron image of saidportion of said cross section, deflecting said electron image over a distance equal to a fraction of its size across an aperture disposed in said plane, subjecting the electrons passing through said aperture to multiplication by secondary emission, and collecting the multiplied electron stream.

RUnoLF BEHNE. WERNER FLECHSIG. 

